It is the goal of this study to determine how retroviruses recognize and enter host cells in order to understand why certain cells are susceptible to infection, and how viral entry may be prevented. The major focus of this work is to identify and characterize receptors for murine leukemia viruses. Recently isolated Chinese hamster-murine somatic cell hybrids will be used to explore a single virus-receptor system against a common genetic background. Initially, the receptor for Moloney glycoprotein gp70 will be isolated from a Chinese hamster-murine somatic cell hybrid which contains mouse chromosome 5, the site of the ecotropic (mouse) receptor. Antibodies to the receptor will be prepared for use in studies of receptor distribution and isolation of related proteins. Identification and characterization of these receptors should provide significant insight into the manner in which retroviruses specifically recognize and penetrate mammalian host cells. In addition, the mechanism of virus penetration into the host cell will be investigated with regard to kinetics of entry, receptor capacity, inhibitors, and other biochemical parameters. The information gained will be utilized to determine whether murine retroviruses enter host cells via the process of receptor-mediated endocytosis used by physiologically important molecules.